Selenium prevents cancer in some cases and fails to prevent it in other cases. Furthermore, in some cases, it may cause toxicity to the host. Therefore, there is a need to understand its cancer preventive mechanism in order to optimize its effectiveness. The following hypothesis will be tested: Redox-active selenometabolites exert their cancer-preventive actions, at least in part, through oxidation of unique cysteine- rich regions and zinc fingers present in protein kinase C (PKC), a known target for a variety of tumor promoters. Importantly, PKC oxidation induced by selenometabolites can be reversed by another product of selenium, the selenoprotein thioredoxin reductase (TR). In preneoplastic prostate cells, a low amount of methylselenol, a volatile selenometabolite, reacts with tumor-promoting peroxides and is converted into seleninic acid, which is better retained and amplified through redox cycling. Seleninic acid inactivates PKC resulting in inhibition of tumor promotion and induction of apoptosis. Normal tissues generally are protected from selenometabolites by TR, whose action is compromised in precancer cells. Advanced tumor cells escape the cancer-preventive actions of selenium via the increased expression of PKC isoenzymes (Se- sensitive 1or Se-resistant 5) and selenoprotein (TR) which naturally occurs in tumor progression. Major emphasis is placed on evaluating the selective cytotoxicity of low tissue-available concentrations (nM) of selenium to promoting precancer cells and distinguishing it from the nonselective global toxicity seen at higher concentrations (5M). Initially, we will use a prostate carcinogenesis model employing carcinogen-initiated human prostate epithelial cells (RWPE-1) grown in culture. The results obtained will then be further validated by extending this model to nude mice fed a diet supplemented with Se-methylselenocysteine or 1,4- phenylene-bis(methylene)selenocyanate. Next, we will use various prostate tumor cells representing early (prostate intraepithelial neoplasia) and advanced stages of malignancy derived from a single clone of RWPE-1 cells to determine the mechanistic basis for the differences in cellular sensitivity to selenium at various stages of prostate cancer. The first aim is to evaluate the causal role of selenium-induced inactivation of PKC1 in the inhibition of tumor promotion in prostate precancer cells. We will also assess the mechanism by which the protective function of selenoprotein (TR) is compromised in precancer cells, allowing the selective toxicity of selenium to occur. The second aim is to assess the significance of selenol action, as amplified by redox cycling with tumor-promoting peroxides, in redirecting tumor promoter-induced signaling from induction of cell survival to restoration of cell death. The final aim is to evaluate how the proposed mechanism that is cytotoxic to precancer cells results in safety to normal cells and development of resistance in advanced prostate cancer cells. These mechanistic studies will help us to understand why in some cases selenium successfully prevents cancer, while in other cases it fails and even can cause toxicity to the host. PUBLIC HEALTH RELEVANCE: Micronutrient selenium is a promising cancer-preventive agent. Some cases it works and in other cases it fails to prevent cancer. Understanding the mechanisms by which it prevents cancer or causes toxicity will allow us to optimize its use as a safe cancer-preventive agent.